I was in a similar position where my 'walkabout' lens wasn't fast enough and I was having to use the built-in flash a lot of the time which wasn't what I wanted. I considered the Canon 17-55mm f/2.8 but the price is offputting as is the need to spend more on the hood.
Instead I bought the new Sigma 17-50mm f/2.8 and have ben very impressed with it. I hadn't considered 3rd party lenses before but this has opened my eyes...even my wife, who is secptical of these things, has commented how my pictures are more 'sharp and colourful and almost 3D'. The Sigma is a solid piece of kit and is very fast and quiet to focus. Unlike the Canon it comes with a lens hood, case and 3 year UK gaurrantee.